In today's society, electrical and electronic devices are plentiful. For example, televisions, personal computers and cellular phones can be found as common household items in many people's homes. These various electronic appliances use a significant amount of energy. Moreover, many of these devices may be battery-powered (or have battery power options) such that battery life or time between battery charges becomes an important device parameter.
Contemporary digital circuits require power, which is typically delivered by a voltage source, to operate in the desired fashion. It is generally known that, within limits, the maximum speed at which a digital circuit can operate depends on the voltage level, where a higher voltage is required for a higher speed. Consequently, a digital circuit requires a sufficient voltage level to operate correctly at its designed speed or frequency.
For a synchronous circuit to work correctly, the clock frequency fclk must fulfill the following criteria:fclk≦1/Tcrit  (1)where Tcrit denotes the longest path propagation time between two storage elements. To operate at a higher clock frequency, Tcrit must be lowered. One way that this can be achieved is by increasing the supply voltage Vdd. This works because the propagation delay tp of a basic complementary metal oxide semiconductor (CMOS) gate is related to the supply voltage Vdd as shown in Equation (2) below.
                                          t            p                    ∝                                    V              dd                                                      (                                                      V                    dd                                    -                                      V                    th                                                  )                            α                                      ,                            (        2        )            where Vth is the threshold voltage of the given CMOS technology, and 1≦α≦2 which is also technology dependent. For an asynchronous digital circuit, no clock is used, and the circuit basically operates at the maximum speed as set by the supply voltage.
One way to fulfill the voltage requirement for a digital circuit is to use a fixed, sufficiently high voltage. However, it is also known that the power P consumed by a digital circuit depends on the supply voltage, where a higher supply voltage leads to higher power consumption as shown by:P=Pstat+Pdyn  (3)where Pstat is proportional to Vdd, and Pdyn is proportional to V2dd. To be power efficient, it is thus desired to use as low of a supply voltage as possible which allows the circuit to still function correctly.
To be able to apply as low a supply voltage as possible, it is useful to know what the voltage needs to be for the circuit to operate. However, the exact relation between supply voltage and the maximum speed of a digital circuit includes partly unknown factors, for example, factors that vary between different physical samples of the same circuit, and factors that vary during use of the circuit due to, e.g., operating temperature. Thus, the lowest overall supply voltage level is usually obtained by some adaptive method during actual use of the circuit.
Various systems and methods have been used to reduce or improve voltage control. For example, such methods include dynamically changing the voltage as a function of the required speed or completely turning off the voltage when the circuit is not in use. Another method relies on the use of a replica of the part of the circuit that limits the speed of the circuit, e.g., the so called “critical path”. In principal, this minimum supply voltage for the replica is determined and then used, possibly with some margin, for the whole circuit. However, these various solutions do not use all of the information available at run time to select the lowest possible voltage level for a given required speed for a digital circuit.
Accordingly, systems and methods for determining and then supplying a lowest possible voltage for a given required speed for a circuit are desirable.